Backflow test for oil concentration

ABSTRACT

The concentration of oil phase fluid in subterranean reservoir is measured by injecting slugs of aqueous liquid containing a preferentially water soluble tracer (i.e., distinctive material) that reacts within the reservoir to form a tracer material that is relatively oil soluble and is partitioned between the oil and water phase fluids.

United States Patent 1191 Hesse et al. 1 Aug. 7, 1973 [54] BACKFLOW TEST FOR OIL 3,623,842 11/1971 Deans 23 23 EP CONCENTRATION [75] Inventors: Robert J. Hesse, North Olmsted, primary Examine, MorriS wolk Ohio; Robert F. Farmer, Baltimore, Assistant Examiner R' Reese Attorney-H. W. 'Coryell et al. [73] Assignee: Shell Oil Company, Houston, Tex.

[22] Filed: Oct. 13, 1971 v 21 Appl. No.: 188,446 [57] ABSTRACT The concentration of oil phase fluid in subterranean U-S. reservoir is measured by injecting lugs of aqueous [51] Int. Cl. G0ln 33/24 id t inin a referentially water soluble tracer Ileld Search 23/230 (i.e., distinctive material) that reacts within the reser- 166/250 voir to form a tracer material that is relatively oil soluble and is partitioned between the oil and water phase [56] References Cited fluids.

UNITED STATES PATENTS 3,590,923 7/1971 Cooke, Jr. 166/252 5 Claims, No Drawings 1 BACKFLOW TEST FOR OIL CONCENTRATION RELATED PATENT APPLICATION The present patent application is related to patent application, Ser. No. 2766, filed Jan. 14, 1970, by C. A. Chase, Jr., and M'. Prats now U.S. Pat, No. 3,690,l67. The priorapplication describes a testing procedure in which fluid is injected into the reservoir in the form of slugs of aqueous liquid that alternately contain and are free of a tracer, or distinctive material, that is selectively water soluble and substantially 'inert within the reservoir. That process is particularly useful with respect to measuring the diffusion capacity.-

of the reservoir. The present process can be used in conjunction with the process described in the'prior application and the disclosure of: the priorapplication is incorporated herein by reference.

BACKGROUND OF THE INVENTION the type descrbed in; the patent application mentioned:

above. I

SUMMARY OF THE INVENTION In accordance with this invention, the concentration of oil phase fluid-in a reservoir that contains. both oil phase and water phase fluid, is measured by injecting an aqueous liquid. solution containing a preferentially water soluble: tracer that is hydrolyzed within the-reservoir to. form atleast one reaction product that is significantly oil soluble and ispartitioned between the oil and water phase fluids, backflowing fluid from the reservoir, and measuring the concentration with time of the injected tracer and its. reaction to product.

In a preferred procedure, the present invention is practiced by an injection of a compound, A, (followed by a slug of water) that reacts irreversibly at a relatively slow rate compared to the injection time such that upon stopping the flow it has produced significant proportion of compounds, B and C. If B partitions with the oil and C does not, upon backflow, S can be calculated by the chromatographic relation:

chemical, A, be organic with a functional linkage that makes it oil insoluble and undergoes a hydrolysis reaction within the reservoir.

There are many advantages tothis approach: a) the rate constant need not be known exactly; b) finding a product that partitions with a large number of oils is easier than finding a product that reacts irreversibly with oils.

' can be varied.

It is desirable'that dispersion and flow control be known as accurately as possible, so that t, and i can be calculated. Calculationsbased on this idea show that if the proper variables are chosen, accurate results are feasible.

A preferred class of' components andreactions comprise: 1

The reaction rates for such reactions are within reasonable limits. Such compounds are B-keto esters in which the length of the hydrocarbon chains in the groups R, R, and R," can be varied. The partitioning properties of the hydrolysis products, particularly the ketones,

vary with. differences in the chain lengths. By varying the chain length of R the oil-water partition coefficient By varying R, the units to which R are attached can be made substantially oil insoluble and water soluble. For example, if R ischosen to be. a. polyethylene glycol group, the polyhydric alcohol is substantially completely insoluble in oil.

By varying-R" (ancl possibly R and R), a desired reaction rate constant can be obtained. The rate constants at which several beat-ketoesters are hydrolyzed at several temperatures. and concentrations have been studied. An attractive ester foruse in the present process is ethyl acetoacetate (EAA), which hydrolyzes as follows:-

The nuclear magnetic resonance spectra, NMR, of EAA, acetone, and ethyl alcohol in water is such that NMR can be used to give the, desired rate data. Cadmium chloride can be used to assign the proton resonance lines and the rate constants can be calculated from the concentration with time of EAA and acetone in the hydrolysis reaction products.

In one preferred procedure an aqueous solution that contains both a readily hydrolyzable beta-ketoester having at least onedecomposition product that partitions between oil and water phase fluids and an inert water soluble tracer material that is substantially oil insoluble i.e., is a substantially exclusively water soluble tracer material is injected into a reservoir that contains both water phase and oilphase fluids. Slugs of aqueous liquids containing such tracer materials are preferably alternated with slugs of similar aqueous liquids devoid of the tracer materials. The injected fluid is allowed to remain within the reservoir until a significant amount of hydrolysis has occurred and is then backflowed and recovered. The concentrations of the oilphase fluid in the reservoir is calculated from the concentrations of the tracer materials and hydrolysis products with time in the fluid that is backflowed and recovered.

With respect to the selectively water soluble tracer material, materials such as tritium or water containing a water soluble radioactive salt or the like can used. The tracer concentration within the well bore is governed by the solution to a convective diffusion equation. A one-dimensional model is assumed in which the flow of carrier fluid is at a constant volumetric rate in the radial direction. The dispersion coefficient is assumed to be a function of the fluid velocity. Following Perkins et al, (Perkins, T.K. and Johnston, CC. (1963), Review of Diffusion and Dispersion in Porous Media, J. Soc. Petr. Engr., March, p. 70.), the functional form is taken as ln dimensionless form, the convective diffusion equation becomes:

where the dimensionless radial coordinate r,, r,,/r,(3

dimensionless time t,, q, t /'r,,2 (4) 1 m IA (1 /rr i/h volumetric injection rate per radian per' unit of height r, arbitrarily chosen scale factor for length By matching solutions of Equation (2) with field test data, estimates of the parameters a,, a and B may be obtained from which (1) and the dispersion parameter ad may be calculated.

Where:

S Residual oil saturation K Dispersion coefficient D, Molecular diffusion coefficient of tracer D Molecular diffusion coefficient of tracer in porous material a lnhomogeneity factor used in correlation of dispersion coefficients d Average particle diameter of porous material u Darcy flow velocity vector d) Mobile fluid fraction B Dimensionless parameter r,, Dimensionless radial coordinate C Dimensionless tracer concentration 2,, Dimensionless time a, Dimensionless parameter a Dimensionless parameter r, Radial coordinate 2 Time 4 Reservoir rock porosity i Injection rate H Height r Arbitrarily chosen scale factor for length a,, Bulk volumetric flow rate EXAMPLE I Core Test A 10-inch Berea sandstone-core,'water permeability K,,, 500 md, porosity 0.2 and pore volume PV 105.4 milliliters was maintained at C and arranged for fluid displacement through the core. It was subjected to inflow of water, n-dodecane and water, to leave a dodecane concentration S 0.37.

A 0.1 PV slug of aqueous liquid solution of hydrolyzable tracer was injected and displaced through the core for about percent of its length. The slug had the following composition:

l. 0.5M ethyl aceto acetate,

2. 0.1M potassium hydrogen phthalate,

3. 0.02M NaCl, and

4. tritiated water to yield approximately 1200 cpm. Both the drive solution and the water with which the core was flooded were 0.025M in NaCl and 0.1M in potassium hydrogen phthalate. The potassium hydrogen phthalate served as a buffer, approximately pH 4, and was simply a precaution. Water in contact with Berea sandstone can reach a pH as high as 8. Sufficient time, 1 1 days, was allowed for ester decomposition to occur. Flow was reversed and effluent solution was analyzed for acetone and tritium content. High resolution NMR spectroscopy served to locate the center of the produced band of acetone.

Hydrocarbon saturation was calculated from the relative positions of the centers of the acetone and tritium bands.

Consider E) A B C where both E and A are partitioned between oil and water and there is also present the nonpartitioning tracer, D.

Let: E ester .4 acetone, and

D tritiated water.

Suppose A is injected into the core and the reaction occurs in place. Now, reverse the flow and monitor the effluent solution; i.e., the above experiment. The experimental data are:

V volume of liquid collected before acetone appears, and

V,, volume of liquid collected before tracer appears. 1n addition, define the following terms:

AV chromatographic holdup of ester,

K,,,,,"'= partition coefficient of ester between hydrocarbon and water, and

K partition coefficient of acetone between hydrocarbon and water. From the equation for the chromatographic relation, for injection,

V AV/V 1 K,,,,,, (S,,/l S") and for production VA/VD l WUMI For brevity, let (S,,,.) (l S") J, then AV k,,,,,, V,,J and fluid from the reservoir, and measuring the concentra- VA V W] KMwAj) A tion with timeof at least one tracer material, the im- I provement which comprises: the quadral'c equauon, injecting an aqueous fluid containing a preferentially K l("' V J K [(501m) VD] (VD VA) 5 I water soluble tracer material which hydrolyzes within the reservoir to yield a reaction product the solution to which is tracer material that is partitioned between the oil We obtained, from experiment, the fdlldlvittg Q and water phase fluids within the reservoir; and

the Parameters: measuring the concentration of the oil phase fluid in VD 471 ml the reservoir by measuring the concentration with time of said tracer materials and the relation be- VA ml tween such concentrations and a chromatographic K 0.462 separation of the tracer materials. K l A 0.096 2. The process of claim 1 in which said tracer con- Th K, taining aqueous fluid is injected in the form of a series 6 S were measure'd between f the of slugs that alternate with slugs of aqueous fluid free aqueous solution used in the core. Substituting into the of the tracer material above expresslon ylekls J 0'767 and 3. The process of claim 1 in which said hydrolyzable ecall that, from material balance, the value of S was tracer material is a betwketo ester of 3 l tz and g giffi-l 4 The process of claim 1 in which said aqueous fluid cu ies ais oo areme t. t worked was g a in 2:31 .Yf 353 g; contains additional substantially exclusively water soluconcentration bands were so diffuse that their centers 3: trace Patel-ml wh'ch ls Substarmauy men w'thm could not be located with certaint e reservmr w is claimed is: y 5. The process of claim 4 in which said hydrolyzable tracer material is a beta-keto ester and said substan- 1. [n a process in which the concentration of phase fluid in a reservoir containing both oil and water exclusll'ely Wale Soluble tracer matenal tium.

phase fluids is measured by injecting an aqueous fluid containing at least one tracer material, backflowing 

2. The process of claim 1 in which said tracer containing aqueous fluid is injected in the form of a series of slugs that alternate with slugs of aqueous fluid free of the tracer material.
 3. The process of claim 1 in which said hydrolyzable tracer material is a beta-keto ester.
 4. The process of claim 1 in which said aqueous fluid contains additional substantially exclusively water soluble tracer material which is substantially inert within the reservoir.
 5. The process of claim 4 in which said hydrolyzable tracer material is a beta-keto ester and said substantially exclusively water soluble tracer material is tritium. 